CELL-CYCLE REGULATION OF THE ACTIVITY AND SUBCELLULAR-LOCALIZATION OFPLK1, A HUMAN PROTEIN-KINASE IMPLICATED IN MITOTIC SPINDLE FUNCTION

Correct assembly and function of the mitotic spindle during cell divis ion is essential for the accurate partitioning of the duplicated genom e to daughter cells. Protein phosphorylation has long been implicated in controlling spindle function and chromosome segregation, and geneti c studies have identified several protein kinases and phosphatases tha t are likely to regulate these processes. In particular, mutations in the serine/threonine-specific Drosophila kinase polo, and the structur ally related kinase Cdc5p of Saccharomyces cerevisae, result in abnorm al mitotic and meiotic divisions. Here, we describe a detailed analysi s of the cell cycle-dependent activity and subcellular localization of Plk1, a recently identified human protein kinase with extensive seque nce similarity to both Drosophila polo and S. cerevisiae Cdc5p. With t he aid of recombinant baculoviruses, we have established a reliable in vitro assay for Plk1 kinase activity. We show that the activity of hu man Plk1 is cell cycle regulated, Plk1 activity being low during inter phase but high during mitosis. We further show, by immunofluorescent c onfocal laser scanning microscopy, that human Plk1 binds to components of the mitotic spindle at all stages of mitosis, but undergoes a stri king redistribution as cells progress from metaphase to anaphase. Spec ifically, Plk1 associates with spindle poles up to metaphase, but relo calizes to the equatorial plane, where spindle microtubules overlap (t he midzone), as cells go through anaphase. These results indicate that the association of Plk1 with the spindle is highly dynamic and that P lk1 may function at multiple stages of mitotic progression. Taken toge ther, our data strengthen the notion that human Plk1 may represent a f unctional homolog of polo and Cdc5p, and they suggest that this kinase plays an important role in the dynamic function of the mitotic spindl e during chromosome segregation.